The present invention relates generally to a measuring method and apparatus, and, more particularly, to a measuring method and apparatus that use shearing interferometry to measure a wave front aberration of a target optical system, such as a projection optical system, which transfers a mask pattern onto an object, and an exposure method and apparatus using the measuring method and apparatus. The inventive measuring method and apparatus is suitable, for example, for measurement of a projection optical system in an exposure apparatus that utilizes extreme ultraviolet (“EUV”) light.
A projection exposure apparatus is used to transfer a pattern on a mask (or a reticle) onto an object to be exposed in manufacturing semiconductor devices, etc., in a photolithography process. This exposure apparatus is required to transfer the pattern on the reticle onto the object precisely at a predetermined magnification. For this purpose, it is important to use a projection optical system having a good imaging performance and reduced aberration. In particular, due to the recent demands for finer processing of semiconductor devices, a transferred pattern is sensitive to the aberration of the optical system. Therefore, there is a demand to measure the wave front aberration of the projection optical system with high precision.
The shearing interferometry is conventionally known as a method for measuring a wave front aberration of a projection optical system (see, for example, Japanese Patent Application, Publication No. 2000-146705). Since the shearing interferometry provides relatively easy alignments and has a wide measurable range of aberration, the improvement of the measurement precision is strongly demanded. The shearing interferometry typically measures differential wave fronts in two orthogonal directions, integrates the entire measured area surface in the shearing directions from a reference point, such as a center point, using these two differential wave fronts, and obtains a shape of the entire target wave front.
One method for measuring the target wave front in the shearing interferometer is to approximately calculate the target wave front by integrating the differential wave front of a component having a period sufficiently larger than the shearing amount, i.e., an offset amount of the wave front, among the spatial wave number components in the target wave front or a low-frequency component of the target wave front. In other words, when the shearing amount is sufficiently smaller than the frequency component of the target wave front, the differential wave front can be considered to be substantially equivalent to the differentiated wave front obtained by differentiating the target wave front. The target wave front can be calculated by integrating the differential wave front measured by the shearing interferometer. However, the recent demand for the improved precision of the projection optical system requires a precise shape of the equivalent wave front including the wave front information of the spatially high-frequency component. The shearing interferometer directly measures only the differential wave front of the target wave front, which is, strictly speaking, not the differentiated wave front. Therefore, a mere integration of the differential wave front causes increased errors in wave number components having periods close to the shearing amount. This is because, as the frequency component of the target wave front becomes higher, the shearing amount cannot be considered to be sufficiently smaller than the frequency component, and thus, the differential wave front cannot be considered to be equivalent to the differentiated wave front. For example, a measured value of a wave number component having a period twice as many as a shearing amount is (2/π)×100=64% of an actually measured value. This problem is solved when measurements cover high-frequency wave number components by reducing a shearing amount. However, in turn, the noise influence becomes problematic due to the reduced output of the differential wave front measured by the interferometer.